New, healthy cells are developed in the bone marrow to replace old or damaged cells. This ensures there is a consistent number of blood cells in the body. With MDS, there is an excessive amount of stem cells that do not fully mature. The immature stem cells crowd the bone marrow making it difficult for new, healthy cells to develop. Overcrowding also pushes immature blood cells into the bloodstream too early. Immature blood cells are not fully formed enough to be functional. The lower levels of healthy cells lead to a weakened immune system, problems problems with oxygen delivery, or problems controlling bleeding depending on the type of blood cells that are affected.

The instruction for cell growth and cell death exists in the DNA of each cell. Primary MDS is the result of damage to DNA. The damage may be the result of genetics, environmental factors like radiation exposure, changes related to age, or a combination of these factors.
Secondary MDS is associated with a history of cancer treatment. Chemo- and radiation therapy can damage the bone marrow and how it functions.

Types of MDS

The World Health Organization (WHO) identifies several types of MDS. Each type has certain characteristics based on how bone marrow or blood cells look under a microscope. They include:

Refractory cytopenia with multilineage dysplasia (RCMD)—This is the most common type of MDS, which is found in nearly half of all cases. RCMD affects at least 2 types of blood cells. In some people, RCMD progresses into acute myeloid leukemia (AML), a disease in which there is an overproduction of a type of white blood cell.

Refractory cytopenia with unilineage dysplasia (RCUD)—Characterized by low numbers of one type of blood cell, and normal numbers of the other two. This type of MDS generally has the best outcome and rarely progresses to AML.

Refractory anemia with excess blasts-1 (RAEB-1)—One or more blood cell types are abnormally low. Some immature blood cells may be present in the bloodstream

Refractory anemia with excess blasts-2 (RAEB-2)—Similar to RAEB-1, except the bone marrow is crowded with more immature blood cells.

Myelodysplastic syndrome, unclassified (MDS-U)—The characteristics of blood and bone marrow cells do not fit into any other classification. This type is not common.

Myelodysplastic syndrome associated with isolated del (5q) abnormality—A specific chromosome in the bone marrow cells is missing. Red blood cell counts are low, but white cell counts are normal. There may be an increased number of platelets.

Revision Information

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.